This invention relates to a voltage-controlled type oscillator and, more particularly, to improvements of a voltage-controlled type oscillator which controls the oscillator frequency of an oscillating section thereof by a collector current of a differential amplifier.
A voltage-controlled type oscillator for controlling the oscillating frequency of an oscillating section by a collector current of a differential amplifier is used in a circuit for oscillating an oscillatory signal of a frequency determined in response to an input control voltage and for adding a vibrato to the signal in an electronic musical instrument.
The voltage-controlled type oscillator of this conventional type includes, as shown in FIG. 1, an operational amplifier OP having a non-inverted input terminal receiving an input control voltage F.sub.c and an inverted input terminal grounded via a resistor Rc. A feedback circuit of the amplifier OP is formed by resistor R and R' and base-emitter paths of the pair transistors Q and Q' and these pair transistors Q and Q', constitute a differential amplifier determining a constant current of common emitters of the pair transistors in response to the input control voltage V.sub.c.
In the meanwhile, a vibrato signal Vib is applied, when necessary, to the input of a voltage-current converting circuit 1 and is thereupon converted to a current signal, which in turn is applied to the base of the transistor Q' of the aforementioned differential amplifier. The transistor Q' thereupon causes its collector current i.sub.c to fluctuate above and below the value determined by the input control voltage V.sub.c.
The collector current i.sub.c of the transistor Q' of the differential amplifier is thus controlled by the input control voltage V.sub.c and the vibrato signal Vib, and the oscillating frequency of an oscillating section 2 is controlled and determined by the collector current i.sub.c. Thus, the oscillating section 2 produced at an output terminal Tout, a signal having a frequency corresponding to the input control voltage V.sub.c and being added with the vibrato signal. In addition, the oscillating section 2 has a circuit for controlling an oscillating frequency in proportion to the charging and discharging time constant of a capacitor (not shown) in such a manner that the discharging current of the capacitor (not shown) becomes the collector current i.sub.c of the transistor Q' of the aforementioned differential amplifier.
In case the emitter current i.sub.e of the transistor Q' is sufficiently large and the base current i.sub.b is in a relation i.sub.b &lt;&lt; i.sub.e in such conventional circuit, the base current i.sub.b can be ignored and the collector current i.sub.c = (i.sub.e - i.sub.b) can be approximated to be equal to the emitter current i.sub.e. Accordingly, this collector current i.sub.c is proportional to the input control voltage V.sub.c, and the oscillating frequency of the oscillating section 2 can be controlled in proportion to the input control voltage V.sub.c. However, if the emitter current i.sub.e becomes small, transistor parameter h.sub.fe of the transistor decreases and, accordingly the base current i.sub.b of the transistor can no longer be ignored in the relation to the emitter current i.sub.e.
Therefore, the collector current i.sub.c of the transistor Q' cannot be approximated to be equal to the emitter current i.sub.e of the transistor Q'. Thus, the conventional circuit has the disadvantage that the oscillating frequency of the oscillating section 2 cannot be controlled in proportion to the input control voltage V.sub.c.